Multiple-input multiple-output (MIMO), referring to multiple antennas at transmitter and receiver sides, is an advanced antenna technology widely applied in wireless communication systems, for example in Universal Mobile Telecommunication Service (UMTS), Worldwide Interoperability for Microwave Access (WiMAX) and Long Term Evolution (LTE).
In LTE, there are two main kinds of transmission schemes in downlink (DL) MIMO: open-loop schemes, e.g. transmit diversity (T×D) and cyclic delay diversity (CDD), and closed-loop schemes, e.g. codebook based precoding and non-codebook based precoding. Reference signals for downlink antenna ports are MIMO enablers, providing channel-dependent measurements, such as rank indicator (RI), precoding matrix index (PMI) and channel quality indicator (CQI), which are then reported by the user equipment in order to assist downlink transmission.
The antenna ports referred to above do not necessarily correspond to specific physical antennas. At least for the downlink, an antenna port can be seen as corresponding to the transmission of a certain reference signal. Any subsequent data transmission from the antenna port can then rely on that certain reference signal for channel estimation for coherent demodulation. Thus, if the same reference signal is transmitted from multiple physical antennas, these physical antennas correspond to a single antenna port. Similarly, if two different reference signals are transmitted from the same set of physical antennas, this corresponds to two separate antenna ports. Stated differently, each antenna port has a unique reference signal associated with it.
Among various practical antenna configurations, geographically-separated antenna ports have been identified as high-priority work item by 3GPP. Geographically-separated antenna ports, such as macro-node with low-power remote radio heads (RRHs) and indoor interleaved antennas ports, are deployed in a way so that the whole set of the antenna ports are not located at a single geographical site, but rather divided into several parts and located at different geographical sites. FIG. 1 illustrates an indoor deployment of interleaved antenna ports, where two antenna ports are distributed at different geographical sites in an interleaved manner.
In the scenario of FIG. 1, every other antenna thus transmits on antenna port 0 (Cell specific reference signal (CRS) port 0) and the other antenna transmits on antenna port 1 (CRS port 1). Advantages of such an interleaved deployment include a reduction in the need of cabling (primarily when upgrading existing passive based distributed antenna systems to support MIMO), as well as a reduction in the number of antennas, compared to deploying two co-located antenna ports per site.
However, measurement results from real networks revealed that spatial multiplexing does not work as expected in the interleaved antenna port deployment. In the contribution R1-111330, “Considerations on Real-Life DL MIMO Aspects”, Ericsson, ST-Ericsson, presented at 3GPP TSG-RAN WG1, Barcelona, Spain, May 9-May 13, 2011, it was shown that close to a site, the two antenna ports undergo a very large difference in receive power, on the order of 35 dB, creating a highly imbalanced, i.e. rank-deficient, MIMO channel.
The measurement results showed an unexpected bad performance. FIGS. 2A and 2B illustrate the received signal strength (Reference Signal Received Power, RSRP) and throughput, respectively, when the user equipment moved from one site to another (e.g. from point A to point B of FIG. 1). FIG. 2A illustrates the RSRP measured at the user equipment, where the peaks correspond to the user equipment being close to one site. FIG. 2B illustrates the throughput of the user equipment. Surprisingly, very low throughput, close to zero, is observed at positions corresponding to RSRP peaks.
The measurement results described above indicate that user equipment is not always able to cope with antenna ports having an imbalance in terms of received power.